The anterior bundle of the MCL is optimally visualized on coronal images. The anterior bundle originates at the anteroinferior surface of the medial epicondyle and courses in an oblique posterior fashion to attach firmly to the medial edge of the coronoid process at the sublime tubercle. The fibers of the anterior bundle are continuous with the sublime tubercle, and any fluid interposed between the distal fibers and the medial aspect of the sublime tubercle is interpreted as a deep articular-sided partial tear with partial stripping of the distal ligament from the sublime tubercle. The fluid signal is perpendicular to fluid within the ulnohumeral joint, resembling the letter “T,” and this appearance has been called the T sign. The anterior bundle is the primary constraint to valgus stress on the elbow
and is one of the main stabilizers of the elbow. The posterior bundle of the MCL is thought to be less functionally important and occasionally is difficult to visualize in the coronal plane.

The common flexor tendon origin is visualized on coronal images arising medial and proximal to the MCL, coursing superficial to the MCL fibers. Tendinosis of the common flexor tendon is visualized as thickening and vague increased T2 (or fat-saturated PD) signal, whereas partial- or full-thickness tears demonstrate fluid signal disrupting tendon origin fibers. Not uncommonly, acute and chronic injuries of the common flexor tendon, often the result of valgus stress, are accompanied by evidence of ulnar neuritis and MCL injuries.

The articular cartilages lining the coronoid and trochlea are optimally visualized in the coronal plane, and chondral degeneration and associated subchondral edema and cystic change are evaluated. The size and location of any loose bodies within the joint secondary to chondral and osteochondral abnormalities are also identified. Fractures of the medial epicondyle, trochlea, and coronoid are also visualized in the coronal plane.

The ulnar nerve courses medial to the distal humerus shaft and posterior to the medial epicondyle. Although axial images provide more detailed characterization of ulnar nerve injuries, high signal and other abnormalities of the ulnar nerve are confirmed in the coronal plane. A normal variant, the anconeus epitrochlearis (a small accessory muscle extending from the medial epicondyle to the medial ulna), occasionally causes ulnar nerve impingement by narrowing the cubital tunnel.

The lateral collateral ligament complex is composed of the RCL, the LUCL, and annular ligament. The RCL and LUCL are optimally visualized in the coronal plane. The RCL originates at the anterior distal aspect of the lateral epicondyle, deep to the common extensor tendon. The RCL fibers insert onto the annular ligament, which circumferentially surrounds the radial head. The origin of the LUCL is also deep to the common flexor tendon and somewhat posterior to the RCL origin along the lateral epicondyle. The LUCL swings posteromedially around the radial head, forming a posteromedial sling for the radius. The LUCL then inserts on the lateral aspect of the proximal ulna at the supinator crest. The most anterior image through the mid-radiocapitellar joint demonstrates the RCL coursing from the lateral epicondyle to the annular ligament surrounding the radial head. The common extensor tendon is superficial to the RCL and originates proximal to the RCL at this image location. On successive coronal images from anterior to posterior, the origin of the LUCL from the lateral epicondyle is visualized, the LUCL is seen swinging around the lateral aspect of the radial head, and the oblique course of the

LUCL posterior to the proximal radius is seen as it courses toward and attaches to the supinator crest on the ulna. Suspected tears and sprains of the RCL and LUCL are confirmed and further characterized in the axial and sagittal planes. Tears of the LUCL predispose to recurrent radial head dislocations. In cases of significant elbow trauma leading to dislocation, it is not uncommon for the RCL, LUCL, and common extensor tendon origin to completely tear off of the lateral epicondyle together as a single ligamentous/tendinous unit.

The common extensor tendon originates from the anterior aspect of the lateral epicondyle and is visualized superficial to and somewhat parallel to the RCL. Common extensor tendinosis and tears are commonly referred to as lateral epicondylitis, or “tennis elbow.” The extensor carpi radialis brevis is the most commonly involved of the extensor tendons. The common extensor tendon is usually visualized on at least three consecutive coronal images from anterior to posterior.

Cartilage covers the articular surface of the radial head and the opposing capitellum in a nearly 180° arc from anterior to posterior. Articular cartilage also extends over the medial aspect of the radial head and the lateral aspect of the coronoid, forming the third articulation of the elbow at the radioulnar joint. The cartilage covering the lateral aspect of the radial head is normally much thinner, and the absence of cartilage in this location

on coronal images should not be misinterpreted as a cartilage defect. Another common cartilage pseudolesion occurs at the lateral aspect of the capitellum, where a normal groove devoid of cartilage between the capitellum and lateral epicondyle may mimic an osteochondral defect. These cartilage surfaces must be examined carefully for the presence of chondral degeneration and associated stress-related subchondral edema and cystic change. Chondral degeneration is most commonly visualized on the opposing surfaces of the medial radial head and adjacent crest at the lateral margin of the trochlea. Osteochondral lesions of the capitellum commonly occur in teenage and young adult throwing athletes, whereas Panner’s disease (capitellar osteochondrosis) occurs in younger children. Fractures of the radial head and neck, not uncommonly occult on plain films, are easily visualized on MR images.

Successive anterior-to-posterior images in the coronal plane display the biceps tendon coursing distally to attach at the radial tuberosity along the medial inferior aspect of the proximal radial shaft. The distal tendon is examined for sprain, tendinosis, or tear. If completely torn, the length of retraction is determined. Hypertrophic bony changes at the radial tuberosity insertion site are seen as indirect evidence of chronic distal biceps tendinosis or tearing. Also, fluid around the distal biceps tendon is compatible with bicipitoradial bursitis rather than tenosynovitis, since the distal biceps tendon has no tendon sheath. The brachialis tendon runs parallel and posterior to the biceps tendon and inserts along the anterior proximal ulna along the ulnar tuberosity. The brachialis is also evaluated for signs of tears and tendinosis.

The triceps tendon is visible from its origin to its insertion on the olecranon. There is normal interposition of fat between the fascicles of the distal triceps tendon, which may produce a heterogeneous appearance to the distal fibers. This should not be misinterpreted as tendinosis as long as individual fibers are still well defined. Tendinosis and strain tend to obscure distinction of individual fibers. Coronal images are particularly useful in determining whether a tear of the distal triceps tendon is complete or partial, and, if partial, the extent and medial-to-lateral location of the tear. The degree of torn tendon retraction can be determined on coronal or sagittal images.

The anterior bundle of the MCLI is most easily found on axial images by first finding the axial image through a small ridge at the most proximal and medial portion of the medial ulna, known as the sublime tubercle. The anterior bundle is seen as a vertically oriented black band of signal lining up along the medial ulna. The anterior bundle can be followed proximally on successive images to where it fans out over its origin on the medial epicondyle. Distal MCL tears are commonly associated with bone marrow edema within the sublime tubercle. Although the coronal plane is probably most useful for evaluating the MCL, the axial and sagittal planes can confirm and further characterize MCL pathology.

Common flexor tendon pathology is visualized in cross-section on axial images. The common flexor tendon origin can be
identified along the anteromedial-most aspect of the medial epicondyle and its course can be followed distally to where it fans out into its various components making up the proximal flexor muscle group. The individual flexor muscles are also delineated and strains, tears, or denervation injuries are identified and localized.

On axial images the ulnar nerve proximal to the elbow courses medially along the medial border of the triceps. It then descends posteriorly at the level of the elbow to run posterior to the medial epicondyle in the cubital tunnel. Distal to the elbow the ulnar nerve ascends between the two heads of the flexor carpi ulnaris muscle. The entire course of the ulnar nerve through the elbow is visualized in cross-section. T2 hyperintensity within the nerve is suggestive of, but not entirely specific for, ulnar neuritis. Swelling and enlargement of the nerve, and inflammatory infiltration of the perineural fat, increase the specificity of the diagnosis. An underlying structural cause for ulnar neuritis is only occasionally found but may include thickening of the cubital retinaculum overlying the nerve, a normal variant anconeus epitrochlearis muscle, neoplasms, bony spurs, posttraumatic deformities, and ganglion cysts. Ulnar nerve compression is sometimes caused by the anconeus epitrochlearis muscle (seen in 11% of the population), which is an anomalous muscle replacing the cubital retinaculum and extending posterolaterally from the posterior medial epicondyle to the medial ulna.

Although usually identified in the coronal or sagittal plane, fractures of the coronoid, trochlea, olecranon, and medial epicondyle are further characterized in the axial plane. Loose bodies within the joint are also confirmed axially. Salter I physeal injuries of the medial epicondyle occur in teenage throwing athletes and are known as “Little Leaguer’s elbow.”

The origins of the RCL and LUCL are visualized in the axial plane along the anterior aspect of the lateral epicondyle. The RCL originates just anterior to the LUCL. From its origin, the short course of the RCL can be followed in cross-section to its insertion on the annular ligament. The longer, oblique course of the LUCL is more difficult to appreciate on axial images but can be identified on successive proximal-to-distal images as the LUCL turns 90° around the posterolateral aspect of the radial head to insert on the supinator crest of the ulna. Tears most commonly occur at the lateral epicondyle origin and are seen as fluid signal disrupting the normal location of origin fibers along the anterior lateral epicondyle. Axial plane images are used to confirm tears suspected on coronal or sagittal images and are sometimes helpful in localizing which components of the lateral stabilizers are involved.

Of the three lateral ligament/tendon structures arising from the lateral epicondyle, the origin of the common extensor tendon is the most posterior and proximal. Tears and tendinosis involving the common extensor tendon are evaluated in cross-section in the axial plane, as are the extensor muscles, which contribute to the common extensor tendon. Strain, tears, or denervation injuries of the extensor muscles, as well as the supinator and brachioradialis muscles, are localized on axial images.

Fractures or osteochondral defects of the radius and capitellum identified in other planes are further characterized in the axial plane. Loose bodies are also identified.

The biceps and brachialis tendons are seen in cross-section, and the biceps tendon should be followed to its distal insertion on the medial inferior aspect of the proximal radius, at the radial tuberosity. In addition, radial tuberosity hypertrophy from chronic tendinosis and distal bicipitoradial bursitis are also evaluated in the axial plane. On images proximal to the elbow, the brachialis tendon begins anterolaterally, whereas the biceps is anteromedial. The brachialis runs posterior to the biceps. As the tendons are followed distally, the biceps courses posterolaterally, whereas the brachialis runs posteromedially. The two tendons cross mid-elbow, with the
brachialis eventually inserting medially on the proximal ulna and the biceps inserting on the proximal radius.

The radial nerve is seen coursing between the brachialis and brachioradialis anterolaterally, splitting into superficial and deep branches near the level of the biceps myotendinous junction. The median nerve runs just medial to the brachial artery, anterior to the brachialis muscle. Evidence of median and radial neuritis is usually seen on axial MR images as denervation (hyperintensity on FS PD FSE images) and/or fatty atrophy of the muscles innervated by the affected nerve. Occasionally, an underlying cause for the neuritis is identified, such as a mass lesion or an anatomic anomaly (e.g., severe bicipitoradial bursitis compressing the median nerve or a ganglion cyst compressing the radial nerve).

The triceps tendon is also imaged in cross-section in the axial plane. It inserts in a U shape along the olecranon. Triceps tendon tears, tendinosis, or strain is evaluated in the axial plane.

On medial-to-lateral sagittal images through the elbow, the common flexor tendon is seen on the most medial images as a hypointense band of signal parallel to the long axis of the ulna, coursing to insert on the medial epicondyle. Tears, strain, or tendinosis of the common flexor tendon can be characterized
on sagittal images, while triangulating on the tendon simultaneously in the coronal and axial planes. The anterior bundle of the MCL is not as well seen on sagittal images but can occasionally be visualized on images just deep to the common flexor tendon. Strain or tears of the flexor muscles distal to the common flexor tendon insertion are also evaluated on sagittal images.

The ulnar nerve can be seen coursing just posterior to the medial epicondyle, making sagittal plane images useful for confirming abnormalities suspected on axial plane images. The ulnar nerve is often visualized on the same sagittal image as the common flexor tendon.

The cartilages lining the coronoid, olecranon, and trochlea are particularly well visualized and characterized on sagittal images. One common pitfall is a normal pseudodefect in the middle of the trochlear notch between the olecranon and coronoid articular surfaces, formed by a normally cartilage-free groove. Fractures of the coronoid process, olecranon, and trochlea are characterized with regard to size and displacement. The size of the coronoid process fracture fragment is important in determining potential elbow instability, and the fracture fragment is measured in the sagittal plane. On successive medial-to-lateral sagittal images near the midportion of the joint, the bone between the trochlea and the distal humeral shaft becomes thin, forming anterior and posterior fossae known as the coronoid fossa anteriorly (which receives the coronoid on flexion) and the olecranon fossa posteriorly (which receives the posterior olecranon on extension). These two fossae are common sites for loose bodies, anterior more often than posterior. In addition, in cases of degenerative arthritis, these two fossae are examined for the presence of hypertrophic bone, which may limit flexion and extension.

On lateral-to-medial sagittal images through the lateral compartment, the common extensor tendon is seen first on the most peripheral lateral slice. It appears as a hypointense band of signal parallel to the radius and can often be visualized along its entire course on one or two images before inserting on the lateral epicondyle. The myotendinous junction of the proximal extensor muscles is also visualized on sagittal images. Strains or tears of the proximal extensor muscles and tendon are well characterized on these images.

The lateral collateral ligament complex is encountered medial to the common extensor tendon. Both the RCL and LUCL are often identified on the same image. The RCL is located just anterior to the LUCL, and the RCL courses distally from the lateral epicondyle, parallel to the long axis of the radius, to insert on the annular ligament fibers surrounding the radial head. The LUCL is located just posterior to the RCL, and the fibers diverge to run posteriorly and distally. The origins
of the LUCL and RCL overlap somewhat on MR images, and occasionally it is difficult to completely distinguish them. The course of the LUCL, as it runs behind the radial head to insert on the supinator crest of the ulna, can be followed on the next three or four successive medial images. The sagittal plane is helpful to confirm or further characterize tears of the lateral ligaments and tendons that are suspected on images obtained in other planes.

The cartilage surfaces covering the radial head and the nearly 180° arc of the capitellum are visualized on sagittal images, as are loose bodies displaced anteriorly or posteriorly in the joint. Fractures of the radial head and capitellum are characterized on these images as well. Similar to the coronoid fossa on the ulnohumeral side, there is a radial fossa forming a small concavity on the anterior surface of the distal humerus, which receives the radial head in flexion. The radial fossa is examined for loose bodies and osteophytes.

On sagittal images through the lateral aspect of the joint, the entire course of the distal biceps tendon can be visualized on one or two images coursing parallel to the distal humeral shaft before curving 90° to insert on the radial tuberosity. Complete tears, partial tears, and tendinosis are characterized with regard to the extent and the location of the involved tendon. In cases of complete tendon rupture, the amount of proximal retraction is measured.

Analogous to the biceps tendon in the lateral aspect of the joint, the full course of the distal brachialis muscle and tendon can be visualized on sagittal images through the medial joint. The brachialis courses parallel and posterior to the biceps tendon, and its distal course and insertion on the anterior proximal ulna are often visualized on the same image as the biceps tendon, or on one image medial to the biceps insertion.

Sagittal plane images through the midline demonstrate the insertion of the distal triceps muscle and tendon on the olecranon. Tendinosis, strain, and tears (including the degree of retraction) can be characterized on these images.

Damage to the medial stabilizing structures is usually caused by chronic microtrauma from repetitive valgus stress. The anterior band of the MCL (Fig. 9.72) is the primary soft-tissue restraint to valgus stress, and it is most at risk during the acceleration phase of throwing (Fig. 9.73).^22,23,24 The MCL may also be injured in posterior dislocation of the elbow. Complete rupture of the anterior bundle of the MCL usually occurs as a sudden event. There may be an acute popping sensation followed by pain and limitation of extension. Clinically, there may be crepitus on palpation of the ligament, and Tinel’s sign may be positive.

Midsubstance MCL ruptures (Fig. 9.74) can be differentiated from proximal avulsions (Fig. 9.75) and distal avulsions (Fig. 9.76). In one study of a large series of surgically treated throwing athletes,¹⁷ midsubstance ruptures of the MCL were the most common (87%), and distal (10%) and proximal (3%) avulsions were found less frequently. In other reports, midsubstance ruptures were not as commonly found.^25,26 Since the fibers of the flexor digitorum superficialis muscle blend with the anterior bundle of the MCL,¹⁶ associated strains of the flexor digitorum superficialis muscle commonly occur when the MCL is injured (see Fig. 9.76). In addition, ulnar traction spurs at the insertion of the MCL on the coronoid process (caused by repetitive valgus stress) have been found in 75% of professional baseball pitchers.²⁷ Chronic degeneration of the MCL is characterized by thickening of the ligament secondary to scarring, often accompanied by foci of calcification or heterotopic bone (Fig. 9.77).¹⁷ The findings are similar to those seen after healing of MCL sprains in the knee, in which the development of heterotopic ossification has been termed the Pellegrini-Stieda phenomenon.

A number of different conditions may occur secondary to the repeated valgus stress to the elbow that occurs with throwing (Fig. 9.78). Medial tension overload typically produces extra-articular injury such as flexor-pronator strain, MCL sprain, ulnar traction spurring, and ulnar neuropathy. Lateral compression overload typically produces intra-articular injury such as osteochondritis dissecans of the capitellum or radial head, degenerative arthritis, and loose body formation. All of these related pathologic processes associated with repeated valgus stress can be assessed with MR imaging.²⁸ The additional information provided by MR imaging can be helpful in formulating a logical treatment plan, especially when surgery is being considered.

Rupture of the MCL is also commonly encountered as a result of posterior dislocation of the elbow.^29,30 After the shoulder, the elbow is the second most common joint to be dislocated.³⁰ The mechanism of posterior elbow dislocation usually involves falling on an outstretched arm. Typically there is rupture of the medial and lateral collateral ligaments as well as the anterior and posterior capsule during posterior elbow dislocation.²² Associated rupture of the common extensor tendon or the common flexor tendon may also occur. The extent of injury secondary to elbow dislocation is well delineated with MR imaging.

Acute injury of the MCL (Fig. 9.79) can be detected, localized, and graded with MR imaging. The status of the functionally important anterior bundle of the MCL complex is best assessed on axial and coronal images (Fig. 9.80). Partial tears of the MCL, which may also occur in pitchers with medial elbow pain, characteristically spare the superficial fibers of the anterior bundle and are therefore not visible with an open surgical approach unless


the ligament is incised to inspect the torn capsular fibers.^31,32 MR imaging, therefore, is particularly important in localizing these partial tears, which are treated with repair or reconstruction. Detection of partial detachment of the deep undersurface fibers of the anterior bundle (Fig. 9.81) may require intra-articular contrast and MR or CT arthrography.³³ The capsular fibers of the anterior bundle of the MCL normally insert on the medial margin of the coronoid process at the sublime tubercle. Undersurface partial tears of the anterior bundle are characterized by distal extension of fluid or contrast along the medial margin of the coronoid, producing the so-called T sign.³³ FS images may show marrow edema due to a stress response at the humeral origin or the coronoid attachment. It is important to differentiate partial tears from an anatomically variant distal MCL insertion (Fig. 9.82).

Typical MR findings include:

After contrast administration, T1-weighted images demonstrate enhancement of the inflamed or injured MCL, and MR arthrograms display the following:

Patients with symptomatic MCL insufficiency usually are treated with reconstruction using a palmaris tendon graft (Fig. 9.88). Graft failure, although unusual, can be evaluated with MR imaging. Other complications include damage to the ulnar nerve and medial antebrachial cutaneous nerve. Lateral compartment bone contusions often are seen in association with acute MCL tears and may provide useful confirmation of recent lateral compartment impaction secondary to valgus instability.

MR imaging is useful for detecting and characterizing acute muscle injury as well as for following its resolution.⁴⁰ Coronal FS sequences are the most sensitive for detecting muscle pathology. The common flexor tendon and MCL should be evaluated carefully for associated tearing when there is evidence of medial muscle strain injury on MR images (see Figs. 9.89 and 9.91). However, abnormal signal intensity within a muscle may simply be due to the effect of a therapeutic injection for epicondylitis, rather than an indication of muscle strain. Increased signal intensity on FS PD FSE sequences may be seen after an intramuscular injection and may persist for as long as a month.⁴¹ Ideally, therefore, steroid injections should be administered after MR imaging to avoid the confounding appearance of the injection on the structures about the elbow.

With MR imaging it is possible to differentiate tendinosis (Fig. 9.92) (secondary to degeneration, microscopic partial tearing, and repair) from macroscopic partial tearing or complete rupture (Fig. 9.93). This distinction is made by identifying fluid signal intensity delineating the presence or absence of tendon fibers on FS PD FSE images. The appearance of medial and lateral epicondylitis about the elbow is similar to the appearance of other common degenerative tendinopathies that involve the attachment of tendons to bone. Similar MR criteria can be used to evaluate the common flexor and common extensor tendons in the elbow, the supraspinatus tendon in the shoulder, the patellar tendon in the knee, and the plantar fascia in the foot. In each of these conditions, there is degenerative tendinosis and a failed healing response that precedes rupture.^39,42,43

The best MR diagnostic clue is thickening and increased signal intensity within the flexor-pronator group (FPG) at the level of the medial epicondyle. In medial tension overload there is often increased signal intensity within the common flexor tendon origin at the medial epicondyle, and the tendon is often thickened as well. Discrete to large irregular tears may also be seen, in which case there is fluid signal intensity within the tendon. Associated common flexor muscle belly strains are also characterized by increased signal intensity. Lateral compression may cause osteochondral injuries of the humeral capitellum with chondromalacia and underlying bone marrow edema or cysts. Loose bodies may also be seen. In skeletally immature individuals there may be avulsion of the medial epicondyle (see discussion of Little Leaguer’s elbow below).

Characteristic MR findings include:

Treatment for medial epicondylitis may be conservative, including limitation of activity, physical therapy, and steroid injections, or surgical. Surgical options include release of the common flexor origin and extirpation of affected tissue; tendon repair; and, in the presence of ulnar neuritis and common flexor tendinosis, transposition or decompression of the ulnar nerve. MR imaging facilitates surgical planning by delineating and grading tears of the common flexor tendon as well as evaluating the underlying MCL and adjacent ulnar nerve. Coronal, sagittal, and axial plane sequences are all useful for assessing the degree of tendon injury. Ulnar neuritis associated with common flexor tendinosis (found in 25% to 50% of patients undergoing surgery for medial epicondylitis) may be difficult to identify clinically. Patients with concomitant ulnar neuropathy have a significantly poorer prognosis after surgery than do patients with isolated medial epicondylitis^44,45 and, as mentioned, require transposition or decompression of the ulnar nerve in addition to débridement and repair of the abnormal flexor tendon.^37,44,45,46 The availability of improved preoperative information from MR studies may reduce the need for extensive surgical exploration in cases in which the MCL is clearly intact on MR imaging. In addition, MR imaging may be useful for problem solving in patients who develop recurrent symptoms after surgery for medial or lateral epicondylitis.

With MR imaging, particularly FS sequences, it is possible to identify soft-tissue or marrow edema about the medial epicondylar apophysis, useful in detecting these injuries before complete avulsion and displacement occur.⁶ The characteristic appearance of the medial muscle-tendon unit varies depending on the age of the patient. In a child, the ossification nucleus of the medial epicondyle may be displaced into the joint (Fig. 9.95), whereas in an adolescent there is more likely to be a crescentic flake of bone avulsed from the margin of the epicondyle (Fig. 9.96).

Key MR findings include:

Although STIR images have a lower spatial resolution, they have a high sensitivity for edema, and characteristic findings include high signal within the fragment and in the parent bone and high-signal edema in the MCL complex, with or without defects.